When producing elastomeric compositions for use in rubber articles, such as tires, power belts, and the like, it is desirable that these elastomeric compositions are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (Tg) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties, such as the compound Mooney viscosity, elastic modulus, tangent delta (tan δ), and the like. Rubber articles, especially tires, produced from vulcanized elastomers exhibiting these improved properties will have reduced hysteresis, better rolling resistance, snow and ice traction, wet traction, tread wear and improved fuel economy for vehicles equipped with such tires.
When silica is employed as a reinforcing filler for rubber, filler dispersion in rubber stocks is a concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration of silica particles can occur after compounding, leading to poor silica dispersion and a high compound viscosity. The strong silica filler network results in a rigid uncured compound that is difficult to process in extrusion and forming operations. Previous attempts at preparing readily processable, vulcanizable silica-filled rubber stocks have focused on the use, during compounding, of bifunctional silica coupling agents having a moiety (e.g., an alkoxysilyl group) reactive with the silica surface and a moiety (e.g., a mercapto, vinyl, methacroyl or sulfide group) that binds to the elastomer. Well known examples of such silica coupling agents are mercaptosilanes, bis(trialkoxysilylorgano) polysulfides, such as bis(3-triethoxysilylpropyl) tetrasulfide (TESPT) and bis(3-triethoxysilylpropyl) disulfide (TESPD), and 3-thiocyanatopropyl trimethoxysilane. These bifunctional silica coupling agents offer excellent coupling between rubber and silica, resulting in rubbers having improved wet ice skid resistance, rolling resistance and tread wear.
However, there are disadvantages to the use of bifunctional silica coupling agents. For example, the high chemical reactivity of the —SH functions of the mercaptosilanes with organic polymers can lead to unacceptably high viscosities during processing and to premature curing (scorch). The tendency of a rubber compound to scorch makes compounding and processing more difficult. Mixing and milling must be done more quickly, yet at lower temperatures (e.g., 120° C. to 145° C.), so that the compound will not begin to vulcanize before it is shaped or molded. Rubber compounds employing TESPT must be mixed at a temperature below 165° C., if an irreversible thermal degradation of the polysulfide function of the coupling agent and premature curing of the mixture are to be avoided. The upper processing temperature limitations of such bifunctional silica coupling agents result in a marked reduction in the mechanical activity of mixing which is essential for an optimum dispersion of the silica throughout the polymer matrix. Therefore, compared with carbon black-filled compositions, tread compounds having good silica dispersion require a longer mixing time at a lower temperature to achieve improved performance, resulting in decreased production and increased expense.
Recent approaches to improving dispersion of silica in rubber compounds have been directed to reducing or replacing the use of silica coupling agents by employing silica dispersing aids, such as monofunctional silica shielding agents (e.g., silica hydrophobating agents that chemically react with the surface silanol groups on the silica particles but are not reactive with the elastomer) and agents which physically shield the silanol groups, to prevent reagglomeration of the silica particles after compounding. For example, silica dispersing aids, such as alkylalkoxysilanes, glycols (e.g., diethylene glycol or polyethylene glycol), fatty acid esters of hydrogenated and non-hydrogenated C5 and C6 sugars (e.g., sorbitan oleates, and the like), polyoxyethylene derivatives of the fatty acid esters have been found to reduce compound viscosity, increase scorch times and reduce silica reagglomeration. Such silica dispersing aids can be used to replace all or part of the bifunctional silica coupling agents, while improving the processability of silica-filled rubber compounds. However, there is an ongoing need to provide alternative silica dispersing aids that improve properties of rubber compounds and of the tire treads and other tire components employing them.